The present invention relates to a back-lighting system for a transmissive electrooptic modulator using the light polarization effect.
Transmissive electrooptic modulators using the light polarization effect, such as, for example, liquid-crystal screens of the twisted nematic type, require for their operation a pair of polarizers which are positioned at the input and at the output of the modulator, respectively. When the transmissive electrooptic modulator is used in direct viewing mode, the polarizer 1 positioned at the input of the modulator is illuminated by unpolarized light 2, as shown diagrammatically in FIG. 1. Since the polarizer only transmits correctly polarized light rays, an absorption of approximately 60% of the total light flux is observed. Moreover, when transmissive electrooptic modulators using the light polarization effect are used in direct viewing mode, a large variation is observed in the contrast and brightness characteristics, depending on the viewing angle.
In order to remedy these drawbacks, it has been proposed to illuminate the electrooptic modulator by collimated light. A system of this type is illustrated diagrammatically in FIG. 2. In this figure, the reference 10 denotes the light source which may be produced by a cold-cathode fluorescent tube or a similar device. The reference 11 denotes a reflector sending the light rays back in the direction referenced x, the reference 12 denotes a light pipe or waveguide, the reference 13 denotes a microprismatic structure and the reference 14 denotes the viewing angle of the light rays at the output. Moreover, a collimation section 15 may be provided between the reflector and the input of the waveguide 12.
With this structure, as illustrated in FIG. 3, the light emitted by the tube 10 is sent back by the reflector 11 in the direction x into the waveguide 12. In this case, the light rays reflected by the reflector 11 pass through the collimation section, producing a first collimation in the xy plane, and are then sent into the waveguide 12. In the guide, the rays emanating from the collimation region 15 are reflected by total reflection off the upper face opposite the microprismatic structure 13, towards this structure, and are then reflected by specular reflection off the said microprismatic structure 13 so as to leave the waveguide approximately perpendicularly through the upper face towards the electrooptic modulator (not illustrated). The path of the light rays is illustrated by the arrows 1 and 1xe2x80x2. With this structure, the light flux is scattered mono-directionally at the output of the electrooptic modulator, thereby making it possible to improve the contrast characteristics.
The object of the present invention is specially, but not exclusively, to improve this type of structure so as to obtain a novel system having a high luminance gain.
The subject of the present invention is therefore a back-lighting system for a transmissive electrooptic modulator using the light polarization effect, comprising at least one means for generating light rays, at least one reflector for sending the light rays back in a first direction x and means for guiding the light rays towards the electrooptic modulator, characterized in that it includes a reflective linear polarizing film positioned in front of the said means for guiding the light rays.
Preferably, the orientation of the reflective linear polarizing film and the means for guiding the light rays are chosen so that the state of polarization of the light remains unchanged from the input to the output of the said means. This is because, from the laws of reflection, a polarization exists which is not disturbed too much by the multiple reflections inside the guiding means. Depending on the coordinate system chosen, if the output of the rays is along z, this preferred polarization is the polarization along y.
With this system, back-lighting is created such that the light rays have a linear polarization favourable to transmission through the polarizer. In addition, the rays emanating from the means for generating light rays or light source, as soon as they have this polarization, keep it throughout the propagation in the means for guiding the light rays towards the electrooptic modulator, the rays which do not have this state being recycled.
According to a preferred embodiment of the present invention, the reflective linear polarizing film, which constitutes a prepolarization and polarization return means, is a thin-layer film which transmits a linear polarization of given optic axis and which reflects all the others. Preferably, this film consists of the film sold by the company 3M under the trademark xe2x80x9cDBEFxe2x80x9d.
According to another characteristic of the present invention, the means for guiding the light rays consist of a waveguide positioned at the output of the reflector so as to transport the light rays in the x direction, one of the faces of the waveguide, parallel to the x direction, having a microprismatic structure which specularly reflects the light rays and extracts them from the waveguide in a second direction, referenced z, perpendicular to the first direction.
Thus, inside the waveguide, the light propagates by total reflection off the upper plane surface of the waveguide and is extracted from the waveguide by reflection off the microprismatic structures. Since the reflections occur mostly in the plane, the linear polarization least affected during the propagation is therefore that along the y axis, i.e. the p polarization for the microprisms.
According to another characteristic of the present invention, the system includes a prismatic film which is oriented so as to be perpendicular to the means for generating light rays and is positioned between these generating means and the reflective linear polarizing film. The function of this film is to increase the luminance at the centre. The film used in this case may be the film sold by 3M under the trademark xe2x80x9cBEFxe2x80x9d or similar films sold by Japanese companies.
Moreover, a half-wave retardation plate is positioned at the output of the means for guiding the light rays. This half-wave retardation plate makes it possible to preserve the linearity of the output polarization and ensures 45xc2x0 azimuthal rotation towards the optic axis of the input polarizer.